Process for bleaching Kraft pulp

ABSTRACT

An improved process for bleaching Kraft pulp is disclosed. The improved process involves the use of hydrogen peroxide to treat brownstock before treatment with chlorine or chlorine containing compounds. The process may also be used to treat recycle pulp.

This application claims the benefit of U.S. Provisional Application No. 60/530,180, filed Dec. 17, 2003.

FIELD OF THE INVENTION

The present invention relates to an improved process for bleaching Kraft pulp and involves the use of hydrogen peroxide in the early stages of such a process—i.e., before the pulp is treated with chlorine or chlorine containing compounds.

BACKGROUND OF THE INVENTION

The production of chemical, or Kraft, pulp involves the cooking of wood chips in a digester at elevated temperature and pressure in the presence of suitable chemicals, such as a mixture of sodium hydroxide and sodium sulfide. The unbleached pulp (brownstock) is removed from the digester, washed and bleached. The purpose of bleaching and washing is to remove lignin from the brownstock and to brighten the pulp for subsequent use in paper making operations. The bleaching takes place in a number of successive steps involving the use of chemicals such as elemental chlorine, chlorine dioxide, sodium hypochlorite, ozone and hydrogen peroxide. Generally, chlorine and chlorine containing compounds have been used in the earlier steps followed by ozone, oxygen or hydrogen peroxide used mainly to brighten the pulp.

SUMMARY OF THE INVENTION

The present invention relates to an improved process in which hydrogen peroxide is used to treat brownstock obtained in a conventional kraft pulping process prior to treatment with chlorine or chlorine containing compounds.

The method of the present invention uses the hydrogen peroxide in the initial treatment of the brownstock following its removal from the digester.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows data from Example 1.

FIG. 2 shows data from Example 3.

FIGS. 3 and 4 show data from Example 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, brownstock from a conventional Kraft pulping operation is treated with hydrogen peroxide prior to treatment with other bleaching agents. When this process is employed, it has been found that the amount of chlorine, chlorine dioxide or other bleaching agents, required in subsequent steps can be reduced without adversely affecting the properties, particularly brightness, of the resulting pulp.

The treatment of the present invention is preferably conducted by including hydrogen peroxide in the water used in washing the brownstock. The peroxide is used alone as the sole bleaching agent and not in combination with other chemicals.

The amount of hydrogen peroxide can vary from 0.05 to 2.0% on pulp weight. In general, preferred results are achieved with peroxide in an amount equal to from 0.1 to 0.8% on pulp weight.

In addition to treatment of brownstock, the present invention can be used to treat recycle pulp such as that obtained in the recycling of newspapers. Here again the recycle pulp is treated with hydrogen peroxide followed by treatment with chlorine or chlorine containing compounds.

The process of the present invention results in pulp having a brightness equal to that obtained in the conventional process while using less chlorine or chlorine containing compounds. It can be conducted in conventional equipment so no capital investment is required and does not require the addition of alkaline chemicals since the brownstock is already alkaline.

In order to describe the invention in more detail, the following examples are set forth:

As is known to those skilled in the art “kappa” relates to the lignin content of the pulp and “brightness” relates to the whiteness of paper made from the pulp.

EXAMPLE 1

A softwood pulp was produced in a digester. After a few stages of washing, the pulp was stored in a high-density storage tank. The pulp kappa number before peroxide treatment was about 30 for several hours prior to the point where peroxide addition was started as shown on FIG. 1.

Hydrogen peroxide was added to the high-density tank at the rate of 10 pounds/ton (0.5% on weight of pulp). Pulp stayed in the tank for approximately two hours at a temperature of 140 degrees Fahrenheit and a pH of 10.5. No caustic was added to the pulp.

The pulp kappa as recorded by the analyzer was about 22 after the peroxide treatment. The pulp was discharged from the tank, washed with additional water and fed to the bleach plant. A reduction of about 20 pounds/ton pulp as equivalent chlorine was noted in the bleach plant. A reduction of about 8 pounds/ton of caustic was also realized in the bleach plant. Pulp quality was maintained and a slight increase in the final pulp brightness was achieved. The kappa and chemical demand as total equivalent chlorine (TEC) are shown in FIG. 1. As can be seen, during the period when peroxide was added at 10 pounds/ton the TEC was reduced. TEC was also reduced when the peroxide was added at 5 pounds/ton. Further reduction in caustic (% NaOH) it also shown in FIG. 1.

EXAMPLE 2

Hardwood pulp was produced in a digester. After a few stages of washing, the pulp was stored in a high-density storage tank. The pulp kappa number before peroxide treatment was about 15 (14.7).

Hydrogen peroxide was added to the high-density tank at the rate of 2 pounds/ton (Example 2-1) (0.1% on weight of pulp) and 6 pounds/ton (Example 2-2) (0.3% on weight of pulp). Pulp stayed in the tank for one hour at a temperature of 140 degrees Fahrenheit and a pH of 10.5. No caustic was added to the pulp.

The pulp was discharged from the tank, washed with additional washer and fed to the bleach plant. The pulp kappa after the peroxide treatment was about 13. A reduction of about 4 pounds/ton pulp of chlorine dioxide (Example 2-1) and about 10 pounds/ton pulp of chlorine dioxide (Example 2-2) was noted in the bleach plant. About 2 pounds/ton caustic was also reduced from the bleach plant in (Example 2-2). Pulp quality and brightness were maintained. The data is shown in the following Table where the results of Example 2-1 and 2-2 are compared to a control in which no hydrogen peroxide was used. TABLE Control 2-1 2-2 H₂O₂ to HD (lbs./ton) 0.0 2.0 6.0 Decker Kappa 14.7 13.6 13.2 Decker Brightness 33.1 32.4 35.1 Bleach Plant: D100 ClO₂ (lbs./ton) 39.3 35.1 28.9 EOP kappa 2.2 2.2 2.3 EOP caustic (lbs./ton) 26.3 26.3 25.6 EOP Brightness 76.0 76.2 75.0 D1 ClO₂ (lbs./ton) 13.5 13.4 13.4 D1 Brightness 89.4 88.9 89.2

EXAMPLE 3

A softwood pulp was produced in a digester. After a few stages of washing, the pulp was stored in a high-density storage tank. The pulp kappa number before peroxide treatment was about 26.

Hydrogen peroxide was added to the high-density tank at the rate of 8 pounds/ton (0.4% on weight of pulp). Pulp stayed in the tank for two hours at a temperature of 140 degrees Fahrenheit and a pH of 10.0. No caustic was added to the pulp.

The pulp was discharged from the tank and fed to the bleach plant. A reduction of about 10pounds/ton pulp chlorine dioxide was noted in the bleach plant. About 4 pounds/ton caustic was also reduced in the bleach plant. Pulp quality was maintained and an increase in the final pulp brightness was achieved. FIG. 2 shows the reduction in chlorine dioxide from about 45 pounds/ton pulp when the hydrogen peroxide was first added to 40 pounds/ton or less over time.

EXAMPLE 4

Softwood pulp of kappa 28.5 was collected from a pulp mill and evaluated in the lab. Two levels of hydrogen peroxide, 0.2 and 0.4% based on the weight of the pulp, were added to the pulp. A base line experiment was also conducted without any hydrogen peroxide. No caustic was added.

Pulp was kept in bags at 140 degrees Fahrenheit for two hours. The pulp consistency was 12%.

The pulp was taken out of the bags, washed and the brightness and kappa number determined. A drop in kappa number and an increase in pulp brightness were noted. The effect was greater for the higher amount of hydrogen peroxide. The results are shown in FIGS. 3 and 4. 

1. A process for bleaching Kraft pulp comprising treating unbleached brownstock pulp with hydrogen peroxide prior to treatment with chlorine or chlorine containing compounds.
 2. A process as in claim 1 wherein the amount of hydrogen peroxide is equal to from 0.05 to 2.0% based on the weight of the pulp.
 3. A process for bleaching recycle pulp comprising treating the pulp with hydrogen peroxide prior to treatment with chlorine or chlorine containing compounds. 